This invention relates to an optical trunk system repeating signals as they are optical and an optical repeater circuit used in this system.
A prior art trunk transmission system utilizing optical signals comprises a 1R optical trunk transmission system utilizing optical amplifiers. The 1R system is constructed to compensate a transmission loss of a power level of a signal light in an optical fiber transmission path, to raise the power level of a signal light to a constant level, and to send out the signal to the optical fiber transmission path. Moreover, the dispersion pre-equalization method called the prechirp method is proposed by N. Henmi and others in a paper reported on Technical Digest PD8 (Post Deadline paper #8) of Inter National Conference on Optical Fiber Communication, 1990. This method comprises the step of dispersion-pre-equalizing a sending signal light by frequency-modulating a semiconductor laser of an optical terminal office, to decrease power penalty caused by wavelength dispersion, to expand a repeater spacing.
Also, the optical trunk transmission method as below is practiced. In this method, as introduced by K. Nakagawa and others in a paper titled "Trunk and distribution network application of Erbium-doped fiber amplifier" reported on pp.198 to pp.208 of "Journal of lightwave technology" second issue, vol.9 by IEEE on February 1991, it is in general to practice regenerative repeating consisting of 3R (reshaping, retiming, regenerating) in each repeater spacing for limiting or reducing dispersion and loss, and in general to practice 3R regenerative repeating after practicing collectively amplification of wavelength by 1R (reshaping) which is constructed by an optical amplifier to dispersion limited or noise limited caused by noise accumulation of the optical amplifier. Also in case of practicing optical trunk transmission of an wavelength-multiplexed signal consisting of N (N is a positive integer) waves, the signal is sent out to an optical transmission path after dividing the waves at each repeater point, repeating by one of the above-mentioned ways at each wavelength and multiplexing the waves of all wavelengths again.
However, in the conventional optical trunk transmission system that repeats and transmits a signal as it is optical utilizing an optical amplifier, there is a problem that transmission length is restricted by wavelength dispersion because of an influence of deterioration of waveform by wavelength dispersion in the optical fiber, although the power level of the signal light is amplified. Moreover, in the prechirp method that compensates the influences by wavelength dispersion in advance and sends out a signal light, there is a problem that the maximum transmission length is restricted because the compensatable wavelength dispersion value is restricted in practice.
Furthermore, even practicing 3R regeneratively repeating, when sending a wavelength-multiplexed signal of N waves, such optical trunk transmission system comprising of dividing N waves at each repeater and regeneratively repeating each wavelength has some problems that N light sources for sending N waves are needed for each repeater, the wavelength of each light source for sending has to be adjusted corresponding to the wavelength of each channel of the optical multiplexing circuit to select extremely high-accurate light sources for sending correspondingly to wavelength of each channel, and they are required extremely high stability for temperature change and age-deterioration. Additionally, there is another problem that control circuits are naturally needed in such system, and the size of such a system including control circuits becomes bigger more than N-times of a conventional single wavelength photonic repeater that does not need any high-accurate wavelength controllers.